Protobuffs compiler take more opts other than just erl_opts

Basically, this issue was brought up by https://github.com/rebar/rebar/pull/150 but it failed to get merged back. I did some improvements on that pull request and Im trying to bringing it up again since I believe it is pretty important for some ppl like me.
Did you change this to be similar to `rebar_abnfc_compiler` and `rebar_neotoma_compiler`? If so, okay, but it's better to use a separate commit with a proper explanation (why).